1. Field of the Invention
The present invention relates to an improved explosion proof forced air electric heater. The heaters are primarily used for heating hazardous environments where the incidences of fire or explosion are increased due to the presence of flammable gases, vapors, or liquids; combustible dust particles, filings, or ignitable fibers. Furthermore, the heaters can similarly be used to heat non-hazardous environments.
2. Prior Art
Explosion proof forced air electric heaters have been on the market since the 1970's under U.S. Pat. No. 4,117,308. The majority of the forced air electric heaters currently in use rely upon a liquid filled heat exchanger. (See U.S. design Pat. No. D 356,367). These heat exchangers are generally comprised of three main components: (1) a steel bottom header; (2) steel tubes with roll formed aluminum fins; and (3) a steel top header, which houses a pressure relief valve. The bottom header contains the electric heating source, which is typically a tubular electric resistant element submerged in a glycol water mixture within the cavity of the bottom header. The prior art's heat transfer process is initiated by supplying the electric heating elements with electricity. The electricity is converted into heat, thereby increasing the temperature of the glycol water mixture to its boiling point, thus creating glycol steam, which rises through the steel tubes and into the top header. The heat is then conducted to the steel tubes and transferred to the roll formed aluminum fins where an air mover forces cool air over the fins to distribute the heat. The heating cycle is repeated when the glycol steam cools and reverts back to the bottom header in liquid form for reheating. The heat exchanger is typically vacuum charged to reduce the resistance exerted upon the glycol steam and to allow for even heat distribution during the cycle. The prior art's safety mechanisms include: the top header's pressure relief valve, which protects the heat exchanger in the event pressure limits are exceeded; and the high limit temperature switch imbedded in the bottom header, which cuts power to the electric heating elements and air mover should the system overheat.
The prior art is typically controlled by a thermostatic switch which monitors the ambient or desired environmental temperature. A call for heat typically activates a switch that engages a mechanical contactor, thereby triggering the electric heating elements and air mover simultaneously. Once the demanded temperature is achieved, the switch disengages the mechanical contactor and immediately turns off the electric heating elements and air mover.
The prior art's typical explosion proof forced air heaters have proven themselves reliable, however, the manner with which they transfer electric heat into the atmosphere and the lack of controllability thereof suggests several key short comings in the current design.